1. Field of the Invention
This invention relates to a diaphragm and, particularly, it relates to a diaphragm using metal materials.
2. Description of the Prior Art
In the case of using metal materials as a diaphragm, various countermeasures are taken for improving the acoustic characteristics thereof. Specifically, since metal materials generally have sharp resonance or lower internal loss, they produce sharp peaks near the high frequency critical point fh or peculiar colorations resulted therefrom to produce harsh sounds. This drawback can be solved to some extent by the use of vibration damping metal materials that is, vibration-damping alloys such as Al-Zn, Mg-Zr, Ti-Ni or by combining metal materials with vibration damping materials, for example, in case of an aluminum substrate, so as to make a composite vibration damping structure by coating or appending vibration damping rubber or resin such as synthetic rubber, natural rubber, foamed urethane or like other elastomer therewith. This vibration damping structure has generally been adopted under consideration not only the vibration isolation effect but also the endurance particularly, improvement in the corrosion resistivity of metal by coating or lamination and view point of the outer looking. The prior art therefor includes coating of resin such as urethane, epoxy, acryl, etc, to the surface of the metal material, and lamination of elastic films such as of olefin, amide, ionomer. However, if the amount of the vibration-damping material is increased with an aim of improving the vibration-damping effect, the thickness and thus the weight are proportionally increased thereby leading to the reduction in the sensitivity.
While on the other hand, improvement in the endurance and increase in the strength are required for metal materials used as diaphragms and, particularly, improvement in the modulus of specific elasticity or increase in the sonic velocity is demanded. However, the improvement in the mechanical strength or the increase in the elasticity is generally in contradiction with the reduction of the resonance described above and it is difficult to simultaneously satisfy both of the requests. Further, increase in the density of the material and thus in the entire weight for improving the strength causes to the reduction in the sensitivity. Although as the prior method of increasing the strength and elasticity includes deposition of metal borides, carbides, nitrides, oxides or the like to the surface of materials by means of CVD, PVD, such as sputtering, plasma welding, ion beams or the like or flame spraying of ceramics are known, they cannot be applied with ease, requiring large-scaled apparatus and highly developed technique. Further, while it has been also considered to improve the strength or the like by compositing them through lamination of different kinds of metals into a clad structure or alloying them with each other, it has not always been quite satisfactory when the relationship with the problems of the vibration-damping as described above, and increase of the weight, productivity, workability and the like are considered.
Referring, for example, to aluminum as the metal material having been used to diaphragms, although it has moderate acoustic physical properties, and the workability, endurance, productivity and cost performance thereof are satisfactory to some degree, there has been a limit for the practical use because of the low internal loss or high resonance and insufficiency in the strength. Accordingly, aluminum is not advantageous in a case where the high frequency critical point fh is intended to be extended toward a higher frequency or where it is intended to suppress the peaks at the high frequency region to thereby flatten the sensitivity.
In view of the above, reduction in the resonance sensitivity and increase in the strength as described above have highly demanded in using the aluminum as the metal material. The situations are identical also in the case of using magnesium, titanium or the like.
As described above, various methods for improving by making composite materials have been adopted in order to solve the problems of metal materials as described above. As one of the typical examples, methods which constitute a honeycomb diaphragm has been proposed. In this method the range of the reproducing frequency band is determined with D/.sigma., where D represents bending regidity and .sigma. represents surface density and, since the bending rigidity D can be increased by the honeycomb structure, the range of the reproducing frequency band can be extended. However, the bending rigidity D has to be more increased in order to further extend the range. Further, it is desired to decrease the surface density .sigma. by selecting the material used for the surface material. In view of the above, it is necessary to reduce the weight and increase the strength of the surface material. Furthermore, in order to suppress the generation of sharp peaks of high sharpness of resonance at higher harmonics mode in the honeycomb diaphragm, it is necessary to improve the internal loss of the surface material, that is, to reduce the resonance as described also above. In addition, reduction in the density is desired in view of the contribution to the increase in the sensitivity.
The situations are identical also in the vibration systems other than the honeycomb diaphragms, where it has been desired for the reduction of the resonance, increase in the rigidity and reduction in the density for the metal materials used for the diaphragm.
While on the other hand, there has been proposed a technique for improving the acoustic characteristics by anodizing aluminum and filling nickel or molten aluminum in the micropores of the alumina layer (refer to Japanese Patent Publications Nos. 13198/1982 and 11553/1982). However, in the proposed technique, diffusing force of filling into the micropores is weak to result in a problem in the close bondability and instability. In the case of nickel filling, a disadvantage of increased density is caused. In addition, there has been also proposed to form many fine pores in the substrate metal such as aluminum and fill the pores with those substances having large internal loss such as synthetic resin or oil (refer to Japanese Patent Publication No. 15156/1980). However, the technique also has a problem in the stability and can not be applied with ease to those materials having micropores such as anidic oxide film. There is also a problem of degradation in the filled synthetic resin or oil. In addition, the density becomes excessively great.